http://portfolio.sticktacular.com/po...p?creation=211

I realize it isn't realistic, but its the best I could find in a 2 minute google search.

Expands then fades out. The cloud doesn't become more dense. Also, wouldn't arc the sparks (from what it looks like, those things have more of an X component than Y even at the very top), those things are going really fast and at that size, air resistance is negligible. So, their velocity should be be straight for the most part.

My two cents.